A bolometer comprises as a main element a component that changes its electrical resistivity or resistance when exposed to different temperatures, this component also called herein a resistance element. The component is mounted so that it absorbs radiation, e.g. visible light or infrared light, the intensity of which is to be detected. The component is connected to or in an electric circuit, sensing the resistance changes, the electric circuit called a read-out electronic circuit.
The performance of infrared bolometers and infrared bolometer arrays for imaging applications can be significantly increased if said component comprises sensing devices based on semiconductor grade materials, typically high purity mono-crystalline materials and structures, which are used on top of read-out electronic circuits and have a high sensitivity and a low noise. The read-out electronic circuits provide a corresponding signal from which also small changes of resistance and thereby of the radiation intensity can be derived.
A large number of infrared imaging applications, such as thermography, fire fighting, automotive night vision and surveillance require infrared imaging arrays having a high performance and preferably also having a low cost. For many of these applications and systems, noise equivalent temperature differences (NETDs) in the order of 20 mK are required. The NETD is a performance parameter of an infrared imaging system and is defined as the temperature difference between two side-by-side blackbodies of large lateral extent that gives rise to a difference in signal-to-noise-ratio of 1 in the electrical outputs of the two halves of the infrared sensor array when the emitted infrared radiation from the two blackbodies illuminate the infrared sensor array. To provide infrared bolometer arrays having such high performance parameters for the mentioned applications, it is also desirable that they can be operated at ambient temperatures, such bolometers called uncooled infrared bolometers, and in an atmospheric pressure environment or at least in an environment having low requirements on an ambient vacuum atmosphere, i.e. that they can be operated at not too small pressures. Thereby, the cost for the vacuum packaging of an infrared bolometer array can be reduced. Hence, a technology is needed to manufacture and integrate infrared detectors that have a high sensitivity and good noise characteristics.
Said main component can, in the case where is comprises a semiconductor material, include one or more PN junctions, such as in one or more diodes or in one or more transistors. In particular the PN junctions can be formed from amorphous materials such as amorphous silicon on top of CMOS based read-out integrated circuit wafers, the read-out circuit components being placed at least partly underneath the bolometer, see P. W. Kruse, “Uncooled Thermal Imaging. Arrays, Systems, and Applications”, SPIE Press, Bellingham, U.S.A, 2001. and L. Dong, R. F. Yue, L. T. Liu, “A high performance single-chip uncooled a-Si TFT infrared sensor”, Proc. Transducers 2003, Vol. 1, pp. 312-315. Amorphous materials are used since they can be deposited on the CMOS wafers without destroying the IC circuits. However, a component comprising such PN junctions suffer from a low sensitivity and high noise characteristics.
Semiconductor structures comprising horizontally placed diodes or PN junctions, i.e. basically semiconductor chips having the PN junctions located parallel to the large surfaces of the chips, such as semiconductor junction devices made from monocrystalline silicon or devices comprising mono-crystalline quantum well (QW) structures, which are made on silicon-on-insulator (SOI) wafers, have a high temperature sensitivity and low noise characteristics, i.e. the electrical resistance thereof changes considerably for small temperature changes and a corresponding generated signal representing the resistance has a considerable swing and low noise, see H. Funaki, H. Honda, I. Fujiwara, H. Yagi, K. Ishii, K. Sasaki, “A 160×120 pixel uncooled TEC-less infrared radiation focal plane array on a standard ceramic package”, Proc SPIE 2009, Vol. 7298, 72980W. However, the read-out electronic circuits of bolometers comprising such semiconductor structures have a very limited functionality since they must be placed beside the resistance component due to the fact that standard CMOS circuitry cannot be manufactured underneath the resistance component. Generally, monocrystalline and/or epitaxially deposited materials requiring high deposition temperatures cannot be deposited on top of integrated circuits without destroying the same.
The performance of uncooled infrared bolometers is discussed in the paper F. Niklaus, C. Vieider, H. Jakobsen, “MEMS-Based Uncooled Infrared Bolometer Arrays—A Review”, Proc. SPIE 2007, Vol. 6836, pp. 0D1-0D15, Beijing, China, which is incorporated by reference herein.
Uncooled infrared bolometers and a method of manufacturing them are disclosed in the published International patent application WO 01/54189, which is incorporated by reference herein. They comprise a membrane structure that includes the resistance element and is arranged at a distance of a substrate carrying the read-out electronic circuits. The resistance element comprises mono-Si, poly-Si or quantum wells based on GaAs as the sensor material and it is geometrically arranged in different ways.